1. Field of the Invention
This invention relates to the field of refractories and, in particular, to the low temperature bonding of refractory aggregates such as used in ramming, castable, vibratable, plastic, mortar and gunning mixes and in refractory bricks or other refractory shapes. It also relates to refractory products having improved cold strengths.
2. Description of the Prior Art
Various materials have been used in refractory products to provide a ceramic bond at temperatures which are too low for a direct bond to be formed. Good examples of these materials are sodium silicate, chromic acid, and various phosphorus compounds. Sodium and potassium carbonates have also been used for this purpose (U.S. Patent to Althans et al, U.S. Pat. No. 234,737). In water based systems these materials dissolve in the water, and upon heating or drying precipitate out on the surface of the particles in the refractory aggregate and bond them together. The purpose of these bonding agents is to give the refractory product strength at temperatures too low (e.g. below about 1200.degree. C.) for a direct bond to develop between the grains of the refractory aggregate. In general, these materials must be dissolved to work properly otherwise they are not distributed thoroughly through the aggregate; very poor results are obtained when these materials are used in dry solid form. Since water can be used only with difficulty in refractories containing a substantial portion of unreacted CaO these bonding agents are not useful in products utilizing aggregates having a high content of free CaO, such as dead burnt dolomite.
It has been disclosed by Steetley Refractory Brick Company of England that the hot strength, in the temperature range of 1200.degree. C. to 1500.degree. C., of bricks made from dead burnt magnesia aggregate can be improved by adding an oxide of an alkali metal, such as lithium, sodium or potassium, or a compound of lithium, sodium or potassium, which yields an oxide on heating to temperatures below 1200.degree. C., and a temporary liquid binder to the magnesia aggregate, pressing the mixture into bricks and then firing the bricks to a temperature above 1200.degree. C. to form a ceramic bond (Steetley British Pat. No. 1,085,841). It is well known in the refractory art that magnesia bricks fired at temperature below those at which ceramic bonding takes place possess very little cold strength. The alkali metal compound preferred by Steetley is sodium carbonate. Steetley uses an aqueous solution, such as a solution of sulphite lye, magnesium sulphate or sulphuric acid as a temporary bonding agent. Magnesia contains little or no free CaO which would react with the aqueous binder with disastrous results, unlike dead burnt dolomite in which the free CaO is usually present in excess of 50% of the total weight of the material.
Steetley British Pat. No. 1,225,966 discloses a modification of the process of the earlier British Pat. No. 1,085,841. This patent discloses that a small amount of calcium oxide or calcium oxide precursor such as calcium carbonate or "stabilized dolomite" may be added to the magnesia aggregate along with the lithium, sodium or potassium oxide before addition of the temporary aqueous binder and firing. Stabilized dolomite is a product obtained by reacting dolomite and serpentine to obtain a mixture of di-calcium and tri-calcium silicates, brown millerite, magnesia and magnesia spinels. It is stated that the amount of CaO added by this procedure should be less than 5% where using a calcium oxide additive not containing silica and less than 10% where part of the CaO is tied up in the form of silicates. Calcium carbonate and "stabilized dolomite" are the preferred additives. Bricks made from this mixture and fired to a maximum temperature of 1650.degree. C. show improved hot strength.
It is also disclosed in U.S. Pat. No. 2,957,752 that the density of magnesium oxide made from magnesium chloride may be improved by mixing the magnesium oxide with a lithium compound which is volatilizable under firing conditions, hydrating the mixture and drying and then firing the mixture in the range of 1200.degree. C. to 1500.degree. C.
Other patents have disclosed the use of lithium compounds as dopants for magnesium oxide coatings and crystals, as a catalyst for forming periclase and synthetic diopside and in ceramic mixtures containing MgO, TiO.sub.2 and ZnO.sub.2. Reference is made to U.S. Pat. Nos. 2,823,134, 3,697,322, 3,476,690 and 4,146,379 and to U.S.S.R. Patent Nos. 487045 and 662531.